Systems and methods for synthesizing gps measurements to improve gps location availability

ABSTRACT

There are situations where GPS signals are received from less than four satellites. In order to improve the GPS location availability, disclosed here are systems and methods for synthesizing GPS measurements, which, together with fewer than four available real GPS signals, can be used to calculate a position fix. In particular, GPS range measurements for lost satellites, which are satellites that were previously tracked but are now not tracked, are synthesized to improve GPS signal availability. The synthesized measurements are used along with real measurements to enable accurate position fix even when GPS satellite availability is poor. Different synthesized measurement generation schemes, depending on whether an INS/DR aiding system is available, are further described herein.

TECHNICAL FIELD

The present disclosure is generally related to global positioning system(GPS) technology and, more particularly, is related to systems andmethods for synthesizing GPS measurements to improve GPS locationavailability.

BACKGROUND

The Global Positioning System (GPS) is a satellite-based radionavigation system. In the GPS system, each GPS satellite, also calledSpace Vehicle (SV) broadcasts time-tagged ranging signals and navigationdata. A GPS receiver tracks the satellites whose signals are within itsfield of view. From these visible satellites a GPS receiver extracts thenavigation data and obtains range measurements from its received GPSsatellite signals. The range measurements are used in a navigationsolution to calculate a position fix of the GPS receiver.

The GPS navigation data contain, but are not limited to, satelliteephemeris, where ephemeris parameters can be used to accuratelycalculate satellite position and velocity. In addition to knowing thesatellite position, the GPS receiver receives the range measurement tothe satellite in order to calculate the position fix. Range measurementsinclude, but are not limited to, two types of measurements—pseudorange(PR) and delta pseudo range (DPR).

A pseudorange is the apparent distance from the GPS receiver to thesatellite. It is calculated by multiplying the speed of light by theapparent transit time, which is the time difference between a signalreception time based on a receiver clock and a signal transmission timebased on a satellite clock. This range is termed pseudorange since thereceiver clock is not synchronized with the satellite clock and thus themeasured range is not necessarily the true range.

A second type of range measurement is the delta pseudo range (DPR),which in some receivers are reported in forms of carrier phase orDoppler frequency measurements. DPR is the measured pseudo range changerate. It is a function of a relative velocity, and a relative clockfrequency drift between the satellite and the GPS receiver. The DPRmeasurement allows the receiver to calculate a receiver velocity and areceiver clock frequency drift, from which a new position fix can thenbe obtained if a previous position is known.

Pseudorange measurements associated with at least four satellites aretypically used to obtain a position fix. This is because the receivertypically not only resolves its 3-D position, but also its clock biassince the receiver clock is not synchronized with the satellite clock.Four pseudorange measurements thus give four simultaneous equationsenough information to calculate its 3-D position and clock bias. If onlythree pseudorange measurements are available, then the receiver cangenerally obtain a 2-D position fix by assuming the receiver's altitudeis known. If less than three pseudorange measurements are available,then the receiver cannot compute the position fix.

Satellite ephemeredes are typically decoded from live signals before thenavigation solution uses the pseudorange data to obtain a first positionfix; however, it takes at least 18 seconds to collect a set of satelliteclock and ephemeris parameters, and in some weak signal environments,correctly decoding ephemeredes is very difficult. Several solutions areavailable to reduce the time to first fix (TTFF). One such solution isthe Assisted GPS (AGPS) technology which uses a GPS assistance server toassist the GPS receiver in obtaining the position fix. In one type ofAGPS operation modes—Mobile Station assisted (MS-assisted), the GPSreceiver sends the GPS measurement data wirelessly to the GPS server forthe GPS server to calculate the receiver position. It is a solution,among others, to the Federal Communication Commission (FCC) Enhanced-911(E911) requirement which mandates that the position of a cell phone tobe available to emergency call dispatchers.

However, in the operation of the GPS receiver, there are many locationswhere the number of satellites available to the GPS receiver formeasurement is less than four. Such situations arise in urban canyons,parking structures, tunnels, or other locations with significant signalblockage. In these situations the GPS server in an AGPS operation modeor the navigation solution in a stand-alone GPS receiver may not be ableto obtain the position fix or may only obtain a very poor fix.

SUMMARY

There are situations where the GPS signals are received from less thanfour satellites. In order to improve the GPS location availability,disclosed here are systems and methods for synthesizing GPSmeasurements, which, together with fewer than four available real GPSmeasurements, can be used to calculate a position fix. In particular,GPS range measurements for lost satellites, which are satellites thatwere previously tracked but are now not visible, are synthesized toimprove GPS signal availability. The synthesized measurements are usedalong with real measurements to enable a more accurate position fix evenwhen GPS satellite visibility and geometry were originally poor.Different synthesized measurement generation schemes, depending onwhether an INS/DR aiding system is available, are further describedherein.

GPS range measurements can be synthesized for stand-alone GPS receiversthat do not have any external aiding sensors or systems or for GPSreceivers where the external sensors or systems do not deliver any validoutputs. Alternatively or additionally, the GPS range measurements canbe synthesized for GPS receivers where external sensors such as inertialnavigation system (INS) sensors or automobile dead reckoning (DR)systems are available to deliver valid outputs.

Other systems, methods, features of the invention will be or will becomeapparent to one skilled in the art upon examination of the followingfigures and detailed description. It is intended that all such systems,methods, features be included within the scope of the invention, and beprotected by the accompanying claims.

BRIEF DESCRIPTION

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, the reference numerals designate corresponding partsthroughout the several views. While several embodiments are described inconnection with these drawings, there is no intent to limit thedisclosure to the embodiment or embodiments disclosed herein. On thecontrary, the intent is to cover all alternatives, modifications, andequivalents.

FIG. 1 is a block diagram that illustrates a system for synthesizing GPSmeasurements;

FIG. 2 is a block diagram that illustrates an embodiment of a navigationdevice 115, such as that shown in FIG. 1;

FIG. 3 is a block diagram that illustrates an embodiment of a GPS signalprocessing system, such as that shown in FIG. 2, which synthesizes GPSmeasurements;

FIG. 4 is a flow diagram that illustrates an embodiment of thearchitecture, functionality, and/or operation of the system, such asthat shown in FIG. 1, which synthesizes pseudo-range (PR) and deltapseudo range (DPR);

FIG. 5 is a flow diagram that illustrates an embodiment of thearchitecture, functionality, and/or operation for estimating PR and DPR,such as that shown in step 430 of FIG. 4;

FIG. 6 is a diagram that illustrates a GPS signal availability scenariothat generally occurs in urban canyon environments; and

FIG. 7 is a block diagram that illustrates an embodiment of a GPS signalprocess system 210, such as that shown in FIGS. 2 and 3.

DETAILED DESCRIPTION

Exemplary systems are first discussed with reference to the figures.Although these systems are described in detail, they are provided forpurposes of illustration only and various modifications are feasible.After the exemplary systems are described, examples of flow diagrams ofthe systems are provided to explain the manner in which GPS measurementsare synthesized.

FIG. 1 is a block diagram that illustrates a system 100 for synthesizingGPS measurements. A simple system 100 comprises a plurality of signalsources 105, 110, 113, 114 and a navigation device 115. Alternatively oradditionally, a more complex system 100, such as an assisted globalpositioning system (AGPS), further comprises a base station 120 and aserver 125. Although only one navigation device 115, one base station120, and one server 125 are shown in system 100, the system 100 caninclude multiple navigation devices, multiple base stations and/ormultiple servers. Alternatively or additionally, the server 125 may beco-located with the base station 120 or with the navigation device 115.

The signal sources 105, 110, 113, 114 include GPS satellites (also knownas space vehicles), among others. The signal sources 105, 110, 113, 114generally orbit above the location of the navigation devices 115 at anygiven time. The navigation devices 115 include, but are not limited to,GPS receivers, cell phones with embedded signal receivers, and personaldigital assistants (PDAs) with embedded signal receivers, among others.The signal sources 105, 110, 113, 114 transmit signals to the navigationdevices 115, which use the signals to determine the location, speed, anddirection of the navigation devices 115.

In an AGPS system, a GPS assistance server 125 assists the navigationdevices 115, such as a Mobile Station (MS) client (e.g., a cellularphone) in obtaining a position fix on the client's position. In oneembodiment of the AGPS, the MS client 115 sends GPS measurements to theGPS server 125, which then calculates the client's position. For anaccurate 3-D position fix the MS client 115 generally receives signalsfrom four satellites 105, 110, 113, 114. However, signal blockage due tothe mobile environment in which the client operates may preventvisibility to four satellites 105, 110, 113, 114. For example, as amobile client 115 moves through the urban canyon, satellites 105, 110,113, 114 may drop out of view, and conversely other satellites 105, 110,113, 114 may come into view. If the GPS measurements to the lostsatellites can be synthesized during the time interval when less thanfour satellites are visible, then the navigation device 115 and/or theGPS server can calculate the 3-D position fix.

FIG. 2 is a block diagram that illustrates an embodiment of thenavigation device 115, such as that shown in FIG. 1. The navigationdevice 115 includes, but is not limited to, sensor(s) 205, a GPS signalprocessing system 210, and a user interface 215. It should be noted thatthe sensor 205 may not be included in some navigation devices 115. Thesensor 205 can include, but is not limited to, inertial sensors thatinclude, for example, micro-electromechanical system (MEMS) sensors,such as, for example, accelerometers and gyroscopes, among others. Ingeneral, accelerometers measure acceleration of their own motion. Theaccelerometer detects specific force which includes gravity and vehicleacceleration.

In general, an inertial navigation system (INS) has accelerometers andgyroscopes accelerometers mounted on multiple axes of a carrier vehicle.The accelerometers measure the vehicle acceleration and the gyroscopesmeasure orientation or angular change rate. In general, the sensor 205can detect the difference between the moving and stationary vibrationsof a vehicle. In particular, the sensor 205 can detect the accelerationand/or the angular change rate of the vehicle and generate a vehiclevibration profile based on the detected acceleration and/or the detectedangular change rate. Based on the sensor measurements, the INS cancalculate the traveled distance and direction change throughintegration.

INS is one kind of dead reckoning (DR) systems. Besides the INS, anotherDR system is, e.g., differential wheel speed sensor method, in which thedistance traveled and the heading change can be derived based on one ortwo pairs of left and right wheel sensors. In general, dead reckoningrefers to a process of calculating location by integrating measuredincrements of distance and direction of travel relative to a knownlocation.

The GPS signal processing system 210 can include, but is not limited to,a GPS receiver, among others. Not only is the GPS receiver capable ofcomputing position fixes by itself without sensors 205, but it can alsobe integrated with the INS/DR. The INS/DR and GPS can be integrated in aloose, tight or deep way. The integrated system can not only eliminatethe error growth problem of an INS/DR but also overcome lots ofdifficulties faced by the GPS receiver in, for example, urban canyonenvironments.

FIG. 3 is a block diagram that illustrates an embodiment of the GPSsignal processing system, such as that shown in FIG. 2, whichsynthesizes GPS measurements. The GPS signal processing system 210 caninclude, but is not limited to, an antenna 305, an application-specificintegration circuit (ASIC) hardware 303 and a tracker and navigator 317.The ASIC hardware 303 includes a radio frequency (RF) front end 310 anda baseband digital signal processing (DSP) 320. The navigation trackerand navigator 317 includes, but is not limited to, a position, velocity,and time (PVT) unit 330 and tracking loops unit 320 that controls theDSP. The tracker and navigator 317 can be, for example, in a form ofsoftware running inside a microprocessor, among others.

The antenna 305 receives GPS signals for visible satellites, and sendsthe received signals to the RF front end unit 310 that down-converts,magnifies, filters, and digitizes the received signals into digitalimmediate frequency (IF) signals 315. Such digital IF signals 315 areinput to the baseband DSP unit 320 that acquires and tracks the receivedsignals and then generates pseudorange measurements 325 according to thereceived GPS signals. In one embodiment, the baseband DSP 320 includes acarrier tracking loop (not shown) that computes an estimated DPR and acode tracking loop (not shown) that computes an estimated PR.

The baseband DSP unit 320 delivers the generated pseudorangemeasurements 325 to the PVT unit 330 that computes a GPS solution orposition fix 335 based on the generated pseudorange measurements 325with or without the aid of an external aiding sensor or system. The PVTunit 330 includes, but is not limited to, a navigation algorithm (notshown), among others, which can include, but is not limited to, aLeast-Square (LS) or Kalman filtering, among others. It should be notedthat in an AGPS system, the server 125 receives the GPS measurementsfrom the navigation devices 115 and calculates a position fix on thereceiver's position.

When signal blockage occurs in an urban environment, the RF signal maybe impaired where the baseband DSP 320 is unable to track the signalsources 105, 110, 113, 114 (FIG. 1) and then fail to generate the GPSrange measurements for the signal sources 105, 110, 113, 114.Accordingly, GPS range measurements can be synthesized at, for example,the baseband DSP 320, PVT Unit 330, or a stand alone GPS measurementsynthesis unit (not shown) between the baseband DSP 320 and PVT Unit330, among others. Operations for synthesizing GPS range measurementsare described in relation to FIGS. 4 and 5.

FIG. 4 is a flow diagram that illustrates an embodiment of thearchitecture, functionality, and/or operation of the system, such asthat shown in FIG. 1, which synthesizes pseudorange (PR) and deltapseudo range (DPR). Beginning with steps 405 and 410, a navigationdevice 115 (FIG. 1) receives GPS signals from signal sources 105, 110,113, 114 (FIG. 1) and calculates the pseudoranges and delta pseudoranges associated with the GPS signals from the signal sources 105, 110,113, 114, respectively. It should be noted that calculated PRs and DPRsand previous PRs and DPRs are stored in memory (not shown) in thenavigation device 115.

In step 415, the navigation device 115 determines whether the navigationdevice 115 receives three (3) or less signal sources. If the navigationdevice 115 receives GPS signals from at least four signal sources, thenavigation device 115, in step 420, calculates the PRs and DPRsassociated with the GPS signals from the at least four signal sources.In step 420, the navigation device 115 and/or the server 125 calculatesa position fix using the calculated PRs and DPRs.

If the navigation device 115 receives the GPS signals from three or lesssignal sources, the navigation device 115, in step 425, determines whichsignal source(s) was lost or became untracked. In step 430, thenavigation device 115 estimates pseudorange and delta pseudo range foreach lost signal source based on the received GPS signals. In step 435,the navigation device 115 and/or server 125 calculates a position fixusing the calculated pseudoranges and delta pseudo ranges as well as theestimated pseudorange and delta pseudo range in a navigation solution.

FIG. 5 is a flow diagram that illustrates an embodiment of thearchitecture, functionality, and/or operation for estimating pseudorange(PR) and delta pseudo range (DPR) for signal sources 105, 110, 113, 114(FIG. 1) that are temporarily invisible or lost, such as that shown instep 430 of FIG. 4. Beginning with step 505, the navigation device 115determines whether the navigation device 115 has external aiding sensorsor systems 205 (FIG. 2), or the navigation device 115 receives GPSinformation associated with the external aiding sensors or systems 205.

The navigation device 115 has a time counter for each satellite, whichrecords the time when the satellite was last tracked. This recorded timecannot be too long from the current time, t₂, the difference of whichare compared with the time threshold. In step 510, if the navigationdevice 115 does not utilize external aiding sensors or systems 205, thenavigation device 115 determines whether the difference between therecorded time and the time, t₂, associated with estimating the PR andDPR passed (e.g., is less than) a time threshold. The DPR is assumed tostay relatively constant over a short time interval; thus, the timethreshold includes several seconds in any case and up to a minute forhigh-elevation satellites if the navigation device 115 is static ormoves with a nearly constant speed.

The time, t₁, generally refers to the last time when the PR and DPRmeasurements, either real or synthesized, are valid. Once a measurementis received or synthesized at time, t₂, it immediately becomes time, t₁,for next epoch. Accordingly, t₂−t₁ is usually very close to 1 second(but not exactly 1 second).

Generally, the DPR is a function of the relative velocity between thesignal source 105, 110, 113, 114 (FIG. 1) and the receiver 210 (FIG. 2)projected along the line of sight, the clock frequency drift of thesignal source 105, 110, 113, 114, and the receiver's clock frequencydrift. It is expressed by

dr ^((k)) ={right arrow over (v)} ^((k)) −{right arrow over (v)}_(rx))·{right arrow over (1)}^((k)) +δf _(rx) −δf ^((k))  (Eq. 1)

where dr^((k)) refers to the DPR of a satellite 105, 110, 113, 114,{right arrow over (v)}^((k)) refers to the satellite velocity, {rightarrow over (v)}_(rx) refers to the receiver velocity, {right arrow over(1)}^((k)) refers to the line of sight unit vector from the receiver 210to the satellite 105, 110, 113, 114, δf_(rx) refers to the receiverfrequency drift, and δf^((k)) refers to the satellite clock frequencydrift.

The receiver 210 is assumed to have a constant velocity motion relativeto the satellite 105, 110, 113, 114. Since the satellite velocity {rightarrow over (v)}^((k)), about 3 kilometers per second, is much largerthan the receiver velocity {right arrow over (v)}_(rx), the smoothmotion of the satellite 105, 110, 113, 114 and its line of trajectorylargely determine the relative velocity between the satellite 105, 110,113, 114 and the receiver 210. Within a short time interval, it isreasonable to assume that the satellite 105, 110, 113, 114 moves with aconstant velocity projected on the line of sight. Therefore, ({rightarrow over (v)}^((k))−{right arrow over (v)}_(rx))·1^((k)) is generallyconstant for a short duration of up to 5 seconds for land vehiclenavigation and longer than 5 seconds for a receiver that is static ormoves with a constant speed. The δf_(rx) and δf^((k)) are alsoreasonably assumed to be stable for this brief duration.

The relationship between PR and DPR is expressed by

pr ₂ ^((k)) −pr ₁ ^((k))≈(t ₂ −t ₁)×dr ₂ ^((k))  (Eq. 2)

where pr₁ ^((k)) and pr₂ ^((k)) are the noise-free pseudorangemeasurements for the same satellite 105, 110, 113, 114 delivered by aGPS receiver 210 at t₁ and t₂, respectively, and dr₂ ^((k)) is thecorresponding noise-free DPR measurement at t₂. An approximationequality used in the above formula is due to the code/carrier phaseionospheric divergence, among others. For example, the ionospheric delaymakes PR measurements longer while the DPR measurements shorter.

If the time difference from step 510 is not small enough or the userdynamic is not constant, then in step 520 the navigation device 115and/or the server 125 stops estimation of the PR and DPR. In step 515,since DPR measures the pseudorange change rate and if DPR is assumed tobe constant over a brief duration and if the receiver 210 loses thetracking of the satellite 105, 110, 113, 114 at t₂, the receiver 210 cansynthesize the kth satellite's pr₂ ^((k)) and dr₂ ^((k)) as:

dr ₂ ^((k)) =dr ₁ ^((k))  (Eq. 3)

pr ₂ ^((k)) =pr ₁ ^((k)) +dr ₁ ^((k))(t ₂ −t ₁)  (Eq. 4)

If the receiver 210 receives the GPS information associated with theexternal aiding sensors or systems 205, the navigation device 115 and/orserver 125 can synthesize GPS measurements using the GPS informationfrom the external aiding sensors 205, such as inertial navigationsystems (INS) or automobile ABS dead reckoning (DR) system. The sensors205 can further include, but are not limited to, Cell ID, Timing Access(TA), Round Trip Time (RTT), Angle of Arrival (AoA), signal strength;with the aid of system such as wireless network of 802.11, Bluetooth; orwith other hybrid technology, and other Global Satellite NavigationSystem (GNSS) signals, such as Galileo and GLONASS. In general, if theposition, velocity and clock can be computed or maintained with the helpof any external aiding sensors or systems, GPS measurements fortemporarily invisible satellites can be synthesized.

The INS and DR sensors 205 generally deliver two pieces of informationabout the motion of the navigation device 115, such as, for example, thedistance traveled and the heading change during a certain period oftime. Alternatively or additionally, an INS/DR system 205 deliversvelocity information in each axis of a coordinate system, either a 3D or2D, depending on the number of sensors. For example, for landing vehiclenavigation, two accelerometers can be mounted and can deliver a 2Dvelocity information in a horizontal plane. The vertical velocity isassumed to be zero.

In another example, if three accelerometers on three perpendicular axesare mounted, then the accelerometers can deliver 3D velocityinformation. Given a 3D velocity {right arrow over (v)}_(rx) andavailable GPS measurements, in step 525, the receiver movement iscalculated as (t₂−t₁)×{right arrow over (v)}_(rx) based on the receivervelocity {right arrow over (v)}_(rx) from the external aiding sensors205. In step 530, given the receiver's position {right arrow over (x)}₁at t₁, the navigation device 115 can update the receiver's position{right arrow over (x)}₂ at time t₂ based on the calculated receivermovement.

{right arrow over (x)} ₂ ={right arrow over (x)} ₁(t ₂ −t ₁){right arrowover (v)} _(rx)  (Eq. 5)

In step 535, based on the updated receiver position {right arrow over(x)}₂, and the satellite position for each available satellite from theephemeris, the line of sight vector {right arrow over (l)} to eachavailable satellite (e.g., satellite k) can be calculated. In step 537,the DPR measurement residual is calculated for each tracked or visiblesatellite k as well as {right arrow over (v)}_(rx) from the externalaiding sensors 205, the actual DPR measurement from the carrier trackingloop, satellite velocity {right arrow over (v)}^((k)) and satelliteclock frequency drift δf^((k)) from the ephemeris based on thecalculated line of sight vector {right arrow over (1)}^((k)) as follows:

δf ^((k)) =dr ^((k))−({right arrow over (v)} ^((k)) −{right arrow over(v)} _(rx))·{right arrow over (1)}^((k)) +δf ^((k))  (Eq. 6)

In step 540, the average of the DPR measurement residuals for all thetracked satellites is regarded as the receiver frequency drift δf_(rx)and is calculated as follows:

$\begin{matrix}{{\delta \; f_{rx}} = {\frac{1}{N}{\sum\limits_{k = 1}^{N}\; {\delta \; f^{(k)}}}}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

where k from 1 to N denotes all the tracked satellites.

In step 545, after obtaining the estimates of the receiver velocity{right arrow over (v)}_(rx) from the external aiding sensors 205, andfrequency drift δf_(rx) at t₂, the navigation 115 synthesizes GPSmeasurements for lost satellites from equation (1) and equation (2) asfollows:

dr ₂ ^((k))=({right arrow over (v)} ^((k)) −{right arrow over (v)}_(rx))·{right arrow over (1)}^((k)) −δf ^((k)) +δf _(rx)  (Eq. 8)

pr ₂ ^((k)) =pr ₁ ^((k)) +dr ₂ ^((k))·(t ₂ −t ₁)  (Eq. 9)

where index k is from N+1 to M, and these satellites were previouslytracked but lost at t₂. For each non-tracked satellite k, satellitevelocity {right arrow over (v)}^((k)), satellite clock frequency driftδf^((k)) and the line of sight unit vector {right arrow over (1)}^((k))from the receiver to the lost satellite at t₂ can be easily calculatedeven though the satellite is not tracked at t₂.

Since the external aiding sensors measurements reflect the actualdynamics of the receiver 210, the navigation device 115 does not need toassume any user dynamic motion model to synthesize the GPS measurements.The navigation device 115 having the external aiding sensors 205 can notonly synthesize both PR and DPR measurements, but can also synthesizemeasurements for lost satellites for a longer duration. Simulations andexperiments showed that the navigation device 115 having the externalaiding sensors 205 synthesizes accurate GPS measurements for satelliteseven after an approximately 60-second outage, for example. Generally,the accuracy of such synthesized GPS measurements and the valid durationfor synthesizing the GPS measurements depend on the accuracy of theexternal aiding sensors 205 and their error accumulation nature. Itshould be noted that the navigation device 115 having the externalaiding sensors 205 generally includes an initial receiver position atprevious time of t₁. If the navigation device 115 does not have theinitial receiver position, the navigation device 115 can calculate theinitial receiver position using steps 510 to 520, which assumes that thenavigation device 115 does not have external aiding sensors.

Though the process in step 430 is intended to improve AGPS locationavailability, the process in step 430 can also be used to improve theavailability of stand-alone GPS receivers. For example, the use of steps525 to 545 to update the average receiver clock frequency drift δf_(rx),and to update the DPR and PR for the lost satellite can be used toinitialize Kalman filter earlier in a standalone GPS navigationsolution. Alternatively or additionally, steps 510 to 520 can be used toobtain 3D Least Squares position fix more quickly in AGPS mode.

FIG. 6 is a diagram that illustrates a GPS signal availability scenariothat generally occurs in urban canyon environments. The GPS signals aretracked according to time of week (TOW) GPS time 605, which is generatedbased on a combination of the current Week Number and the time of week.The TOW refers to the number of seconds into the week ranging from 0second to 604800 seconds and is counted from midnight Saturday/Sunday onthe GPS time scale.

In this example, during the six seconds from 420387 seconds to 420392seconds, there were only three or even two space vehicle measurementsavailable at certain seconds. The GPS signals associated with spacevehicles 1 and 23 are available as first and second sets 610, 615 ofspace vehicle measurements. However, a third set 620 of space vehiclemeasurement is available from both space vehicles 11 and 20. There areno available GPS signals for a fourth set 620 of space vehiclemeasurement. Although losing track of space vehicle 11, the navigationdevice 115 tracked a new space vehicle, e.g., space vehicle 20, from420388 seconds to 420389 seconds. Based on the space vehicle 11measurements at 420388 seconds, the navigation device 115 can synthesizemeasurements 625 for space vehicle 11, for example, at 420389 seconds.Together with the synthesized measurements of space vehicle 11, a totalof four satellite measurements is available where the navigation device115 and/or the server 125 can calculate a 3D position fix at 420389seconds.

FIG. 7 is a block diagram that illustrates an embodiment of the GPSsignal process system 210, such as that shown in FIGS. 2 and 3. The GPSsignal process system 210 includes a processor 710, a network interface720, memory 730, and non-volatile storage 740. Examples of non-volatilestorage include, for example, a hard disk, flash RAM, flash ROM, EEPROM,etc. These components are coupled via a bus 750. The memory 730 includesa navigational solution manager 760 that facilitates synthesizing GPSmeasurements and processing a navigational solution based on thesynthesized GPS measurements. Operations of the navigational manager 760can be described in detail in relation to FIGS. 4 and 5. The memory 730contains instructions which, when executed by the processor 710,implement at least a portion of the methods and systems disclosedherein, particularly the navigational solution manager 760. Omitted fromFIG. 7 are a number of conventional components, known to those skilledin the art that are unnecessary to explain the operation of the device210.

The systems and methods disclosed herein can be implemented in software,hardware, or a combination thereof. In some embodiments, the systemand/or method is implemented in software that is stored in a memory andthat is executed by a suitable microprocessor (μP) situated in acomputing device. However, the systems and methods can be embodied inany computer-readable medium for use by or in connection with aninstruction execution system, apparatus, or device. Such instructionexecution systems include any computer-based system,processor-containing system, or other system that can fetch and executethe instructions from the instruction execution system. In the contextof this disclosure, a “computer-readable medium” can be any means thatcan contain, store, communicate, propagate, or transport the program foruse by, or in connection with, the instruction execution system. Thecomputer readable medium can be, for example, but not limited to, asystem or propagation medium that is based on electronic, magnetic,optical, electromagnetic, infrared, or semiconductor technology.

Specific examples of a computer-readable medium using electronictechnology would include (but are not limited to) the following: anelectrical connection (electronic) having one or more wires; a randomaccess memory (RAM); a read-only memory (ROM); an erasable programmableread-only memory (EPROM or Flash memory). A specific example usingmagnetic technology includes (but is not limited to) a portable computerdiskette. Specific examples using optical technology include (but arenot limited to) optical fiber and compact disc read-only memory(CD-ROM).

Note that the computer-readable medium could even be paper or anothersuitable medium on which the program is printed. Using such a medium,the program can be electronically captured (using, for instance, opticalscanning of the paper or other medium), compiled, interpreted orotherwise processed in a suitable manner, and then stored in a computermemory. In addition, the scope of the certain embodiments of the presentdisclosure includes embodying the functionality of the preferredembodiments of the present disclosure in logic embodied in hardware orsoftware-configured mediums.

It should be noted that any process descriptions or blocks in flowchartsshould be understood as representing modules, segments, or portions ofcode which include one or more executable instructions for implementingspecific logical functions or steps in the process. As would beunderstood by those of ordinary skill in the art of the softwaredevelopment, alternate embodiments are also included within the scope ofthe disclosure. In these alternate embodiments, functions may beexecuted out of order from that shown or discussed, includingsubstantially concurrently or in reverse order, depending on thefunctionality involved.

This description has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit thedisclosure to the precise forms disclosed. Obvious modifications orvariations are possible in light of the above teachings. The embodimentsdiscussed, however, were chosen to illustrate the principles of thedisclosure, and its practical application. The disclosure is thusintended to enable one of ordinary skill in the art to use thedisclosure, in various embodiments and with various modifications, asare suited to the particular use contemplated. All such modificationsand variation are within the scope of this disclosure, as determined bythe appended claims when interpreted in accordance with the breadth towhich they are fairly and legally entitled.

1-25. (canceled)
 26. A method for synthesizing global navigationsatellite system (GNSS) measurements, the method comprising: receiving,by a navigation device, a GNSS signal from a signal source; calculatingpseudoranges (PRs) and delta pseudo ranges (DPRs) associated with theGNSS signal; identifying the signal source as a lost signal source whenthe GNSS signal is not being received; estimating PRs and DPRs for thelost signal source based on previously calculated PRs and DPRs of thelost signal source; determining a position of the navigation deviceusing dead reckoning; determining a validity of the estimated PRs andDPRs based on the dead reckoning; and calculating a position fix usingthe calculated PRs and DPRs, and the estimated PRs and DPRs in anavigation solution based on the determined validity of the estimatedPRs and DPRs.
 27. The method of claim 26, including determining thevalidity of the estimated PRs and DPRs by comparing an elapsed timesince the estimated PRs and DPRs where estimated to a predetermined timeduration determined based on an accuracy of the dead reckoning.
 28. Themethod of claim 26, including determining the position of the navigationdevice using at least one dead reckoning sensor coupled to a vehicle inwhich the navigation device is installed, the at least one sensortracking the position of the navigation device as the vehicle travels.29. The method of claim 28, including determining the validity of theestimates based on an accuracy of the at least one sensor used in thedead reckoning.
 30. The method of claim 26, including determining theposition of the navigation device using the dead reckoning beginningfrom a previously known position fix of the navigation device.
 31. Themethod of claim 26, including determining the position of the navigationdevice using an inertial navigation system (INS) for dead reckoning, theINS including accelerometers and gyroscopes coupled to a vehicle inwhich the navigation device is installed.
 32. The method of claim 26,including determining the position of the navigation device using adifferential wheel speed system (DWSS) for dead reckoning, the DWSSincluding wheel speed sensors coupled to at least two wheels of avehicle in which the navigation device is installed.
 33. A globalnavigation satellite system (GNSS) receiver comprising: an antennaconfigured to receive a GNSS signal from a signal source; and a basebandprocessor configured to: calculate pseudoranges (PRs) and delta pseudoranges (DPRs) associated with the GNSS signal, identify the signalsource as a lost signal source when the GNSS signal is not beingreceived, estimate PRs and DPRs for the lost signal source based onpreviously calculated PRs and DPRs of the lost signal source, determinea position of the navigation device using dead reckoning, determine avalidity of the estimated PRs and DPRs based on the dead reckoning, andcalculate a position fix using the calculated PRs and DPRs, and theestimated PRs and DPRs in a navigation solution based on the determinedvalidity of the estimated PRs and DPRs.
 34. The GNSS receiver of claim33, wherein the baseband processor is configured to determine thevalidity of the estimated PRs and DPRs by comparing an elapsed timesince the estimated PRs and DPRs where estimated to a predetermined timeduration determined based on an accuracy of the dead reckoning.
 35. TheGNSS receiver of claim 33, including at least one dead reckoning sensorcoupled to a vehicle in which the navigation device is installed, the atleast one dead reckoning sensor tracking the position of the navigationdevice as the vehicle travels during dead reckoning.
 36. The GNSSreceiver of claim 35, wherein the baseband processor is configured todetermine the validity of the estimates based on an accuracy of the atleast one dead reckoning sensor.
 37. The GNSS receiver of claim 33,wherein the baseband processor is configured to determine the positionof the navigation device using the dead reckoning beginning from apreviously known position fix of the navigation device.
 38. The GNSSreceiver of claim 33, further including an inertial navigation system(INS), wherein the baseband processor is configured to determine theposition of the navigation device using the INS for dead reckoning, theINS including accelerometers and gyroscopes coupled to a vehicle inwhich the navigation device is installed.
 39. The GNSS receiver of claim33, further including a differential wheel speed system (DWSS), whereinthe baseband processor is configured to determine the position of thenavigation device using the DWSS for dead reckoning, the DWSS includingwheel speed sensors coupled to at least two wheels of a vehicle in whichthe navigation device is installed.
 40. A system for synthesizing globalnavigation satellite system (GNSS) measurements comprising: a GNSSreceiver configured to: calculate pseudoranges (PRs) and delta pseudoranges (DPRs) associated with the GNSS signal, identify the signalsource as a lost signal source when the GNSS signal is not beingreceived, estimate PRs and DPRs for the lost signal source based onpreviously calculated PRs and DPRs of the lost signal source, determinea position of the GNSS receiver using dead reckoning; and an aidingserver configured to: receive the PRs and DPRs and the estimated PRs andDPRs from the GNSS receiver, determine a validity of the estimated PRsand DPRs based on the dead reckoning, calculate a position fix using thecalculated PRs and DPRs, and the estimated PRs and DPRs in a navigationsolution based on the determined validity of the estimated PRs and DPRs,and transmit the position fix to the GNSS receiver.
 41. The system ofclaim 40, wherein the aiding server is configured to determine thevalidity of the estimates by comparing an elapsed time since theestimated PRs and DPRs where estimated to a predetermined time durationdetermined based on an accuracy of the dead reckoning.
 42. The system ofclaim 40, including at least one dead reckoning sensor coupled to avehicle in which the GNSS receiver is installed, the at least one sensortracking the position of the GNSS receiver as the vehicle travels usingdead reckoning, wherein the GNSS receiver is configured to determine thevalidity of the estimates based on an accuracy of the at least onesensor used in the dead reckoning.
 43. The system of claim 40, whereinthe GNSS receiver is configured to determine the position of the GNSSreceiver using the dead reckoning beginning from a previously knownposition fix of the GNSS receiver.
 44. The system of claim 40, furtherincluding an inertial navigation system (INS), wherein the GNSS receiveris configured to determine the position of the GNSS receiver using theINS for dead reckoning, the INS including accelerometers and gyroscopescoupled to a vehicle in which the GNSS receiver is installed.
 45. Thesystem of claim 40, further including a differential wheel speed system(DWSS), wherein the GNSS receiver is configured to determine theposition of the navigation device using the DWSS for dead reckoning, theDWSS including wheel speed sensors coupled to at least two wheels of avehicle in which the GNSS receiver is installed.